The TiBolt takes Parker-style refills, which thankfully is a pretty standard format that many brands offer. Schmidt offers a finer tip in their P 900 refill, which IMO runs a little dryer than the Easyflow M tip refill that comes in the TiBolt. You can also use Fisher Space Pen refills with their Parker adapter (little piece of plastic that you stick on the end), though I have not tried that yet in my own TiBolt. Lastly, Ohto makes some awesome needle-point refills in this style, that offer finer writing. I've been meaning to try one of these out in the pen, but am waiting until the ink runs out first.
Both the Writing and EDC communities are always on the lookout for new and exciting metal pens. If you know of any particular brands you'd like to see offered, make sure to let us know by starting a poll or posting up here in Talk. :)
I'm working on a way to offer a comprehensive database on compatible refill types and the pens they fit, so hopefully that can be made available sometime in the near future as we work more in this new Talk feature. I try to include information on refill types in Specs of all Writing pen drops, and I'll do my best to make sure we include that information more consistently in the future!